1. Field of the Invention
The invention relates to a superconducting rectifier for the conversion of a relatively low current from an alternating current power supply into a relatively high direct current, essentially consisting of a transformer, at least the secondary coil of which is constructed in superconducting form, at least two superconducting switching means for the passing of the alternating current induced in the secondary transformer coil in one direction through a load, and electronic control and regulating switching means.
2. Related Art
A superconducting rectifier of this type, in particular for a flux pump, is described inter alia in two articles by L. J. M. van de Klundert and H. H. J. ten Kate entitled "On fully superconducting rectifiers and flux pumps"--a review--part I: "Realised methods for pumping flux" in Cryogenics, 21, 1981, pages 195-206 and part II: "Communications modes, characteristics and switches" in Cryogenics, 21, 1981 pages 267-277, in particular pages 275 and 276 for the switching means. A rectifier of this type is used for the conversion of the relatively low current from an alternating current supply source into a relatively high direct current with which, inter alia, N.M.R. (Nuclear Magnetic Resonance), also known as M.R.I. (Magnetic Resonance Imaging), systems can be fed. See the article by M. Wood entitled "Superconducting magnets by N.M.R. imaging and in vivo spectroscopy", which appeared in the proceedings of the ICEC 10, July/August 1984, Finland.
To make it possible to generate said high direct current, the secondary side of the superconducting rectifier may in principle comprise two electric circuits and in particular an electric circuit to make it possible to maintain the direct current generated and an electric current to make it possible to increase the direct current generated in steps. To this end each of the two electric circuits is opened or closed alternately by the respective superconducting switching means, which means that, for example, during operation as a superconducting rectifier, the current is continuously controlled to flow through one or the other of the electric circuits, i.e. to commutate. The manner in which said commutation takes place can be classified as resistive commutation and inductive commutation.
In the case of resistive commutation the resistance of the electric circuit in which the current is flowing at that instant is increased by means of the respective known superconducting switching means in a manner such that the current decreases to zero or nearly zero and is taken over by the other electric circuit which at that instant is in the superconducting state. Although resistive commutation is a relatively simple method of commutation, it is subject to a number of drawbacks which are an obstacle to a widespread application. Thus, the maximum current change per unit time, the so-called dI/dt value in the connected circuit, also forms the limit of the maximum current strength which can be achieved with resistive commutation. Said maximum dI/dt value determines in particular the limit above which the circuit will leave the superconducting state and depends in particular on the superconducting wire used. From the known design and material data it is possible to calculate how large said dI/dt value may be as a maximum in a particular circuit. However, this can also be determined by measurements in the respective circuit. The energy diSsipation compared with inductive commutation will also be relatively high because for each commutation a quantity of energy in the electric circuit in which the current decreases to zero is dissipated in the circuit.
The resistance in the electric circuit must further comply with two opposite requirements. During pumping, i.e. in the currentless state of the electric circuit, the resistance must be sufficiently high to obtain as low a dissipation as possible. For commutation, on the other hand, it is precisely a low resistance which is desired to keep the current change per unit time, or the dI/dt value, within the limits imposed during the commutation. The reason for this is that if the current change per unit time is too large, said dI/dt value will become so large that the conductors leave their superconducting state.
Said disadvantages of resistive commutation will not occur in inductive commutation because in this case, by means of a voltage change applied to the primary transformer side, a secondary current is induced in the electric circuit such that the latter causes the current flowing therein to decrease to zero while the associated time interval can at the same time be adjusted so that the maximum dI/dt value will not be exceeded.
In order to achieve this, the instantaneous current in the secondary circuit is now measured and the voltage change on the primary transformer side is dimensioned in time and magnitude in a manner such that the current induced on the secondary side as a result thereof completely or virtually completely corresponds to the measured instantaneous current. If the induced current is precisely equal to the instantaneous current, the commutation loss will be zero. In practice, however, this is not always readily achievable and some commutation loss will therefore occur. In the event of a faulty current measurement and/or feed back, even the maximum permissible dI/dt value may nevertheless again be exceeded in the extreme case, as a result of which the conductors leave their superconducting state.
A considerable disadvantage in the case of inductive commutation is therefore that means must be present to make it possible to determine the instantaneous current in the secondary circuit as accurately as possible and that means must be present to convert the magnitude of the measured current into a primary voltage change corresponding as precisely as possible thereto. In general, said means comprise complicated electronic circuits.